Generally, in order to compensate density irregularity among pixels caused by illumination irregularity of a reading-purpose light source, a scanner device stores dark reference data obtained when an image signal output from an image sensor reading an image while a reading-purpose light source is turned off is converted into digital data and white reference data obtained when an image signal output from the image sensor reading a white reference image is converted into digital data and compensates density irregularity among pixels of digital data converted from an analog signal output from the image sensor by an analog/digital (A/D) converter based on a shading correcting function. Usually, the shading means mismatching in conversion characteristic between an image signal and original luminance of an object image caused by irregularity in sensitivity of an imaging element and vignetting of a camera optical system of an image processing device.
Patent Document 1 below is known as a citation list for the technique of the scanner device having the above-described shading correcting function. Patent Document 1 discloses a technique including an illumination unit, a lens array, a photoelectric conversion member that converts light from the lens array into an electric signal, a white shading correcting circuit and a black shading correcting circuit that are used to correct the electric signal from the photoelectric conversion member, a storage unit that stores a plurality of correction data respectively used in the white shading correcting circuit and the black shading correcting circuit, and a control unit that controls the illumination unit, the photoelectric conversion member, the white shading correcting circuit, the black shading correcting circuit, and the storage unit. Here, the control unit can reduce an influence caused by a change in temperature by selecting correction data among a plurality of correction data and controlling the white shading correcting circuit and the black shading correcting circuit so that the electric signal is corrected based on the selected correction data.